The known prior art can be summarised in the following papers which are discussed more fully below.    [1] McNeill, J. D., 1999, Principles and application of time domain electromagnetic techniques for resistivity sounding, Technical Note TN-27, Geonics Ltd.    [2] Zhdanov, M. S., and Keller, G. V., 1994, The geoelectrical methods in geophysical exploration: Elsevier    [3] Eaton, P. A., and Hohmann, G. W., 1989, A rapid inversion technique for transient electromagnetic soundings: Physics of the Earth and Planetary Interiors, 53, 384-404.    [4] Strack. K.-M, 1992, Exploration with deep transient electromagnetics: Elsevier    [5] Christensen, N. B., 2002, A generic 1-D imaging method for transient electromagnetic data: Geophysics, 67, 438-447.    [6] Strack, K.-M., 1985, Das Transient-Elektromagnetische Tiefensondierungsverfahren angewandt auf die Kohlenwasserstoff- und Geothermie-Exploration, in: Ebel, A., Neubauer, F. M., Raschke, E., and Speth, P., (Hrsg.), Mitteilungen aus dem Institut für Geophysik und Meteorologie der Universität zu Köln 42.    [7] Cheesman, S. J., Edwards, R. N., and Law, L. K., 1990, A test of a short-base-line seafloor transient electromagnetic system: Geophysical Journal International, 103, 2, 431-437.    [8] Cairns, G. W., Evans, R. L. & Edwards, R. N., 1996. A time domain electromagnetic survey of the TAG hydrothermal mound, Geophys. Res. Lett., 23, 3455-3458.    [9] Cheesman, S. J., Edwards, R. N., and Chave, A. D., 1987, On the theory of sea-floor conductivity mapping using transient electromagnetic systems: Geophysics, 52, 204-217.    [10] Yu, L., Evans, R. L., and Edwards, R. N., 1997, Transient electromagnetic responses in seafloor with triaxial anisotropy: Geophysical Journal International, 129, 300-306.    [11] Eidesmo, T., Ellingsrud, S., MacGregor, L. M., and Constable, S., Sinha, M. C., Johansen, S., Kong, F. N., and Westerdahl, H., 2002, Sea Bed Logging (SBL), a new method for remote and direct identification of hydrocarbon filled layers in deepwater areas: First Break, 20, 144-152.    [12] MacGregor, L. M., Constable, S., and Sinha, M. C., 1998, The RAMESSES experiment-III. Controlled-source electromagnetic sounding of the Reykjanes Ridge at 57 45N: Geophysical Journal International, 135, 773-789.    [13] MacGregor, L. M., Sinha, M. C., and Constable, S., 2001, Electrical resistivity structure of the Valu Fa Ridge, Lau basin, from marine controlled source electromagnetic sounding Geophys. J. Int., 146, 217-236.    [14] Ziolkowski, A., Hobbs, B. A., Andrieux, P., Rüter, H., Neubauer, F., and Hördt, A., 1998. Delineation and monitoring of reservoirs using seismic and electromagnetic methods: Project Number OG/0305/92/NL-UK, Final Technical Report to European Commission, May 1998.    [15] Wright, D. A., Ziolkowski, A, and Hobbs, B. A., 2001, Hydrocarbon detection with a multi-channel transient electromagnetic survey: Expanded Abstracts 71st SEG Meeting, 9-14 September, San Antonio, p 1435-1438.
Conventionally time domain electromagnetic investigations use a transmitter and a receiver, or a transmitter and a number of receivers. The transmitter may be a grounded dipole (electric source) or a wire loop or multi-loop (magnetic source) and the receiver or receivers may be grounded dipoles (electric receivers—recording potential differences or electric fields) or wire loops or multi-loops or magnetometers (magnetic receivers—recording magnetic fields and/or time derivatives of magnetic fields). The transmitted signal is usually formed by a step change in current in either an electric source or in a magnetic source.
Known prior developments include (1) a methodology frequently termed TDEM and often taken to imply a magnetic source and a magnetic receiver, (2) the Long Offset Time-Domain Electromagnetic Method (LOTEM) developed for land surveys, (3) time domain electromagnetics in the marine environment (university of Toronto/Scripps Institution of Oceanography), (4) Sea Bed Logging (SBL) using single frequency electromagnetic measurements in the marine environment (Scripps Institution/Southampton Oceanography Centre/Electromagnetic Geophysical Services Ltd.), and (5) our own previous work on multi-channel transient electromagnetic (MTEM) measurements made in collaboration with the University of Cologne, Deutsch Montan Technologie, and Compagnie Generale de Geophysique. These known developments are discussed more fully below.    (1) The TDEM method is exemplified by commercial equipment such as PROTEM from Geonics Ltd., SMARTem from ElectroMagnetic Imaging Technology Pty Ltd (EMIT), UTEM from the University of Toronto and PATEM, a pulled-array from the University of Aarhus. These systems use magnetic sources and magnetic receivers in central loop, coincident loop, offset loop, or borehole configurations and as a consequence delineate conductive rather than resistive targets. They measure voltage induced in the receiver coil at a number of times (referred to as gates) after the transmitter current has been switched off [1]. A decay curve is then formed which is modelled either directly or through the use of various apparent resistivity measures such as early time and late time apparent resistivity [2], or imaged using a rapid inversion scheme [3]. The modelling approach uses a small number of parameters and makes assumptions about the turn-off characteristics of the source, for example that it is a perfect step function or a perfect ramp. TDEM methods all fail to recognise the importance of measuring the system response and instead put much effort into generating a transient signal with as small a turn-off time or ramp turn-off time as possible. The systems and associated software do not determine the earth's response function as defined in the present invention.    (2) The LOTEM method (whose principal researchers are Vozoff, Strack and Hördt), and a similar system developed at the Colorado School of Mines, uses a large dimension electric source, typically 1-2 km long with electric and magnetic receivers placed several kilometer from the source. It is designed for land surveys. Decay curves measured by the receivers may be converted to various apparent resistivity curves. The decay or resistivity curves are modelled using a small number of parameters taken to represent sub-surface conditions beneath the receivers only. The collation of transformed curves from adjacent receivers forms an image representation.     The method includes consideration of a measurement of the system response. It is recommended ([4], p154) that this be performed either in the laboratory, or in the field at the beginning of the survey. LOTEM defines the system response as the response due to a delta-function input, which, it is admitted ([4], p49), cannot be achieved in practice. Instead, a square wave is input and the resulting output differentiated. In reality it is not possible to input an exact square wave either. Usually only one system response is obtained, determined as the average of a statistical representative number of transmitted pulses ([4], p68). An assumption is made that switching characteristics do not vary under load ([4], p155).     Most interpretation methods in the literature are based on a knowledge of the step response. This is impossible to obtain without a deconvolution of the measured data which is stated to be inherently unstable [5]. LOTEM recommends that either apparent resistivity curves are obtained after time-domain deconvolution using an iterative scheme [6] or that synthetic data from modelling is convolved with the system response before comparison with the measured data. A rule of thumb is that this should be done when the length of the system response is more than one third of the length of the transient ([4], p52).     The LOTEM method fails to recognise the importance of measuring the system response for each source transient in the field, and fails to recognise that the decay curves are a function of all the intervening material between the source and corresponding receiver where the induced currents flow.    (3) The University of Toronto sea-floor EM mapping systems (principal researchers: Edwards, Yu, Cox, Chave and Cheesman), consist of a number of configurations including a stationary electric receiver on the sea floor and a towed electric transmitter, and a magnetic source and several collinear magnetic receivers forming an array which is towed along the sea-floor. In early experiments, the system response was measured in free space and was convolved with the theoretical impulse response of a simple model of the sea-water and underlying earth in order to model the measured data [7]. In later experiments, for the case of an electric source, the measured current input to the transmitter is convolved with the impulse response of the receiver, again measured in free space, and then with the impulse response of a model to give a synthetic signal for comparison to that measured [8]. No receivers are placed near the transmitter to determine the system response under load.     The group have developed an extensive library of analytic solutions and recursive numerical schemes for the response of simple geological models to a step change source. The models invariably have a small number of parameters and interpretations of measured decay curves are based on this modelling approach [9], [10].     Their technique fails to recognise the importance of measuring the system response for each source transient and using this to deconvolve the measured transients to obtain the estimated earth impulse response functions.    (4) Sea Bed Logging (SBL) is a realisation of the CSEM (controlled source electromagnetic) method and has been developed by Electromagnetic Geoservices Ltd (EMGS), a subsidiary of Statoil, in conjunction with the University of Cambridge, University of Southampton, and Scripps Institution of Oceanography [11]. It comprises a number of autonomous two-component electric receivers in static positions on the sea floor and an electric source towed approximately 50 m above the sea floor. The receivers remain in their positions on the sea floor recording continuously until instructed to pop up for recovery at the sea surface at the end of the survey. The source (DASI—deep-towed active source instrument) is a 100 m long horizontal electric dipole [12]. Electrodes spaced along the source dipole are used to monitor the transmitted fields. These enable the receiver data to be normalised by the source dipole moment for comparison with modelling results [13]. Unlike the above transient systems, in the SBL technique the source transmits at only one frequency which the operators optimise to the target under investigation [11]. The method relies on the towed movable source creating data for several source-receiver separations and these data are interpreted by modelling. The method does not involve a transient source and takes no account of the system response.    (5) The University of Edinburgh, the University of Cologne, Deutsch Montan Technologie, and Compagnie Generale de Geophysique collaborated within the European Commission THERMIE Project OG/0305/92/NL-UK (which ran from 1992 to 1998) to obtain multi-channel transient electromagnetic (REM) data in 1994 and 1996 over a gas storage reservoir at St. Illiers la Ville in France. The experiment is described in detail in the Final Technical Report to the European Commission, entitled “Delineation and Monitoring of Oil Reservoirs using Seismic and Electromagnetic Methods” [14]. The project had two objectives: first, to develop a method to detect hydrocarbons directly; and second, to monitor the movement of hydrocarbons in a known reservoir. Neither of these objectives was achieved.     Ziolkowski et al. [14] and even Wright et al. [15] failed to recognise the importance of measuring the system response for each source transient.